1. Field of the Invention
This invention relates to a droplet forming apparatus which causes injection liquid generally called ink to discharge and fly as droplets through an orifice by imparting a thermal action to the liquid.
2. Description of the Prior Art
Among the various known recording systems, the so-called ink jet recording method which is a non-impact recording system substantially free of noise during the recording and which enables recording to be effected on plain paper at high speed without requiring the fixation treatment is accepted as a very useful recording system. About this ink jet recording method, various systems have heretofore been proposed and improvements have been made and some ink jet recording systems have already become commercially available while, on the other hand, some ink jet recording systems are undergoing the efforts to put them into practice.
The ink jet recording method effects recording by causing small droplets of recording liquid called ink to fly on various action principles and causing them to adhere to a recording member such as paper or the like.
In this ink jet recording method, use is usually made of an apparatus provided with a recording head having a discharge orifice through which ink may discharge and fly as small droplets and an inflow opening through which the ink may flow in. There are various types of such apparatus depending on the differences in the system for forming small droplets of ink.
For example, one of these types is such that ink is supplied under pressure or under natural supply condition (such as the supply condition utilizing the capillary phenomenon) from an ink supply tank into a predetermined chamber and a voltage is applied between the ink in the chamber and an electrode installed forwardly of the discharge orifice to cause the ink to electrostatically discharge through the discharge orifice.
In another type of ink jet apparatus, ink is caused to discharge and fly as ink droplets by mechanical vibration. That is, this type of apparatus is such that the volume of the chamber into which the ink is supplied is varied by mechanical vibration of a piezo vibratory element in accordance with a signal, whereby the ink is caused to discharge as small droplets. The specific construction thereof is disclosed in U.S. Pat. No. 3,747,120, IEEE Transactions on Industry Applications Vol. IA-13, No. 1, January/February 1977, etc.
A specific example of the droplet forming apparatus for application to the above-described ink jet recording method is already disclosed in U.S. Pat. No. 3,878,519. The droplet forming apparatus disclosed therein may be summarized as follows:
An apparatus for forming droplets at a substantially constant breakoff point and with substantially uniform distances from each other from a liquid stream including:                means to supply the liquid stream through an opening or the like; and        means to selectively alter the surface tension of spaced segments of the stream to form droplets at substantially uniform distances from each other and of substantially uniform size, said selectively altering means being applied to each of the spaced segments of the stream as it passes a predetermined portion of its path to initially reduce the surface tension of each of the spaced segments before random break up of the stream into droplets would occur after the stream exits from the opening.        
As the “means to selectively alter the surface tension of spaced segments of the stream to form droplets at substantially uniform distances from each other and of substantially uniform size” in such an apparatus, there are specifically a “high intensity light source” and “heating means” proximate to the discharge opening.
This apparatus for forming droplets is not of the type which uses all of the ink supply including the liquid pressurizing means for causing the liquid stream to discharge, the deflection means for droplets and the formed droplets and therefore, requires a gutter for collecting unnecessary ones of the droplets and thus, it is difficult to make the entire apparatus compact. Also, in this apparatus, the degree of the force forming the droplets is originally weak and therefore, there is only obtained insufficient uniformity of the droplet diameter. Further, in the same apparatus, unless strict adjustment of the liquid pressurizing force is effected in the ink supply, it is not possible to provide uniformity of the diameters of the droplets, constant discharge speed thereof and uniformity of the discharge direction thereof.